In recent years, video streaming service (video delivery service) via networks is becoming widespread. In the video streaming service, especially, a contents delivery method called progressive download type (hereinafter referred to as “PDL type”) is widely used. In the PDL type video contents delivery method, a server divides a video content into small pieces called chunks, and delivers the chunks. A client downloads the chunks and playbacks the contents of the chunks. In most cases, since the server stores multiple types of encoded data each of which is obtained by encoding an original video data at different encoding rates, the client can select the encoding rate of the chunk to be retrieved at beginning of playback or during playback.
In selecting the encoding rate, if a higher encoding rate is selected, higher resolution of video image can be retrieved. However, since an amount of data in each chunk becomes bigger if a higher encoding rate is selected, a playback stall is likely to occur. On the other hand, if a lower encoding rate is selected, a quality of video image is degraded. But since an amount of data in each chunk becomes less if a lower encoding rate is selected, a playback stall is less likely to occur. That is, since there is a tradeoff between the encoding rate and the playback stall, it is required to establish an encoding rate selecting method to keep balance between the encoding rate and the playback stall. But the optimal encoding rate selecting method is not definite.
Note that the encoding rate refers to the number of bits used for an encoded video image per second. The encoding rate can be regarded as the same as a rate for delivering encoded data (delivery rate). Also, the encoding rate and the delivery rate may be called “bit rate”.
To solve the problem, a method of selecting the encoding rate based on quality of experience (QoE) is proposed (see Non-Patent Document 1 for example). In the proposed method, a network provider utilizes a model for estimating quality of application and QoE based on past network quality information, to determine the delivery rate so that QoE becomes highest. The network provider makes a recommendation of the determined delivery rate to a service provider.